I have superficial mould growth on my loft timbers due to what I believe is lack of ventilation. I live in a mid terrace with large loft conversion and it seems there are no eaves vents coupled with insulation stuffed down between rafters by previous owner of home. Getting some vent tiles fitted on either side of the roof to provide ventilation. Once this is in place, what is the best way to treat the mould growth. Picture below if it helps.

the cuprinol products are very good once the timber is fully dry (so it soaks in). I usually use Cuprinol Green.

you should wear plastic gloves, eye protection, hat etc. If you turn the cuffs of the gloves upwards they will catch drips and stop them running up your arm. Wash very thoroughly including hair, and launder clothes afterwards.

You can get a coarse airless Cuprinol sprayer (like a garden sprayer but a different plastic so the solvents do not damage it) but in this case I would use a very effective mask and skin protection (these are poisonous products). Protect all electrical cables and water tanks, remove insulation to the opposite end of the loft; do not let the spray get into water tanks, lamps or switches.

see also advice about loft condensation at the top of the lofts forum.